Magnetic recording media comprising a non-magnetic support having thereon a magnetic layer comprising a binder having dispersed therein a magnetic powder, such as ferromagnetic iron oxide powder, Co-doped iron oxide powder, CrO.sub.2 powder, and a ferromagnetic alloy powder have widespread applications as video tape, audio tape, and magnetic discs.
Short wave recording has recently been introduced to meet the demand for an increased recording density. For example, the recording wavelength for 8 mm-video tape has reached 0.54 .mu.m. With this tendency, there has arisen a problem of so-called thickness loss on reproduction, that is, reproduction output is reduced as a function of increasing magnetic layer thickness.
In order to cope with this problem encountered in short wave recording, magnetic recording media using a thin film of a ferromagnetic metal have been put to practical use which have a very small thickness due to use of formation methods such as vacuum deposition techniques. Such metal-deposited recording media suffer little thickness loss and attain a very high reproduction output. However, production of such thin film magnetic metal-deposited recording media by vacuum evaporation of a metal on a non-magnetic support is less suited to mass production as compared with so-called coated type magnetic layers formed by the conventional coating techniques involving dispersions of ferromagnetic powders in a binder system. In addition, the metallic film is less reliable for long-term use because of susceptibility to air oxidation.
Therefore, it has been attempted to instead reduce the thickness of a magnetic layer formed by various manipulations of the conventional coating technique to thereby increase reproduction output. However, as the thickness of a magnetic layer is decreased to about 2 .mu.m or less, the surface properties of a support are apt to strongly influence the surface properties of the magnetic layer resulting in deterioration of electromagnetic characteristics.
In order to reduce the thickness of a magnetic layer to minimize thickness loss of magnetic properties thereby achieving a high output while excluding the adverse influences of a support surface, it has been proposed to provide a thick non-magnetic layer between a non-magnetic support and a thin magnetic layer. For example, U.S. Pat. No. 2,819,186 discloses a magnetic recording medium comprising a support having thereon a hard and brittle magnetic layer having a magnetic substance content of 85% by weight or more and a thickness of not more than 0.25 mil as an upper layer and a soft and flexible non-magnetic lower layer having a higher thickness than the upper magnetic layer. JP-A-62-154225 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a magnetic recording medium having a magnetic layer thickness of 0.5 .mu.m or less with a subbing layer provided between the magnetic layer and the support containing carbon black as a conductive fine powder and having a thickness greater than the magnetic layer so as to prevent surface resistance of the magnetic layer from increasing. JP-A-62-222427 discloses a magnetic recording medium comprising a support having thereon a subbing layer containing an abrasive having an average particle size of from 0.5 to 3 .mu.m and a 1 .mu.m or less thick magnetic layer containing a ferromagnetic powder, in this order, in which a part of the abrasive in the subbing layer projects through the magnetic layer so as to serve for cleaning of a magnetic head. Thus, it has been suggested to provide a non-magnetic lower layer immediately adjacent the support to reduce the thickness of a magnetic layer thereby to achieve high-density recording and, at the same time, to incorporate into the lower non-magnetic layer additives such as carbon black for static charge prevention or an abrasive for improvement in cleaning characteristics or durability.
However, conventional techniques for producing these magnetic recording media having a lower non-magnetic layer and an upper magnetic layer involve complicated processes. For example, such processes comprise first coating a non-magnetic layer on a non-magnetic support, then drying the non-magnetic layer, and then, if desired, followed by calendering, and thereafter coating a magnetic layer thereon. However, problems have been identified with these conventional techniques.
For instance, reduction in thickness of a magnetic layer is achieved either by reducing the application quality or by using an increased amount of a solvent in a magnetic binder coating composition to reduce the ultimate film concentration. Yet, when the former approach is taken, drying of the coating transpires too quickly before sufficient leveling can occur to leave surface defects such as streaks or traces of coating pattern, resulting in very poor yield. On the other hand, when the latter approach is taken, a thin coating composition provides a coating film with many voids, resulting in shortage of packing of a ferromagnetic powder or insufficient film strength.
In order to overcome these problems, it has been proposed to form a non-magnetic layer as a lower layer and a thin coat of a highly concentrated magnetic coating composition by a simultaneous coating system. For example, JP-A-63-191315 discloses a magnetic recording medium having a lower non-magnetic layer and an upper magnetic layer formed by simultaneous coating, in which the lower layer has a thickness of 0.5 .mu.m or more and contains no polyisocyanate.
Extensive studies have hitherto been given to the above-described simultaneous coating system or successive wet coating system, called wet-on-wet coating, for formation of a plurality of magnetic layers. However, the same techniques cannot be applied to the lower non-magnetic layer to obtain satisfactory results. That is, where a lower non-magnetic layer and an upper magnetic layer are formed by wet-on-wet coating, disturbances occur in the interface between the upper and lower layers, causing pinholes or run-away of the magnetic coating composition.
Further, although a thick non-magnetic layer formed beneath a magnetic layer eliminates the influences of the surface roughness of the support, the problem of wearability against a recording head or durability is left unmitigated. The poor wearability or durability of conventional magnetic recording media having a non-magnetic lower layer appears attributable to curing of the lower layer comprising a thermosetting resin as a binder for reasons that the magnetic layer formed thereon is brought into contact with a head or other members without cushioning and that the magnetic recording media having such a cured lower layer lacks the desired flexibility.
This problem might be resolved by using a non-curing (thermoplastic) resin as a binder in the lower layer. However, when a magnetic layer is coated on a dry lower layer containing such a non-curing resin, as in the conventional technique, the lower layer is swollen with the organic solvent of the magnetic coating composition, giving undesired influences, such as turbulence of the magnetic coating composition, leading to impairment of the surface properties of the magnetic layer and deterioration of electromagnetic characteristics.
Furthermore, a magnetic coating composition must be diluted with a relatively large quantity of a solvent before it can be coated to a dry thickness of not more than 1.0 .mu.m. Such a diluted coating composition is susceptible to agglomeration. Further, orientation of a ferromagnetic powder is apt to be disturbed during drying due to evaporation of the large quantity of organic solvent. When the medium has a non-continuous form, for example, if it is a magnetic disc, adequate performance properties may still be obtainable to some extent even in using such a thin magnetic coating composition. However, with respect to those media having a continuous form, such as magnetic tapes, although the purpose of thickness reduction is accomplished, it is difficult to obtain sufficient electromagnetic characteristics because of deteriorated orientation and deteriorated surface properties. In addition, many voids are produced during drying, resulting in poor film strength in running. If the amount of the diluting organic solvent is decreased in order to improve orientation properties and to minimize voids, the coating stability would be deteriorated, leading to formation of many pinholes and an increased production of defective media.
On ther other hand, it is known that performance properties of digital recording media may be improved by reducing the thickness of a magnetic layer. Thickness reduction is effective, in principle, but gives rise to production problems. That is, coating defects such as pinholes and coating streaks occur, and a sufficient yield cannot be reached. Further, since calendering effects would be reduced with thickness reduction, the resulting magnetic layer has poor surface properties and unsatisfactory electromagnetic characteristics.
It is suggested to overcome these problems by simultaneously forming a relatively thick non-magnetic layer as a lower layer and a thin magnetic layer of 1 .mu.m or less in thickness as an upper layer, followed by calendering. To this effect, incorporation of non-magnetic abrasive particles or fillers into the lower layer has been proposed as disclosed in JP-A-62-22242 and JP-A-2-257424. However, when a magnetic layer and a non-magnetic layer are simultaneously coated, and the magnetic substance in the upper layer is orientated, the two layers are mixed at the interface due to the rotary motion of the magnetic substance in a magnetic field. As a result, the surface properties and orientation become insufficient, with a failure to obtain sufficient electromagnetic characteristics.
It has been proposed to provide a non-magnetic and conductive intermediate layer containing graphite flakes to improve orientation of the magnetic powder in the upper layer as described in JP-A-55-55438. Although an improvement in orientation can be achieved by this proposal, graphite itself has no film reinforcing effect, and the resulting recording medium lacks in durability. Incorporation of an inorganic powder having a Mohs hardness of 5 or more into the non-magnetic layer has also been proposed as disclosed in JP-A-60-125926. Similarly, it has been proposed to provide a non-magnetic reinforcing layer containing acicular oxalate particles to improve orientation of the magnetic powder in the upper layer as disclosed in JP-B-58-51327 (the term "JP-B" as used herein means an "examined published Japanese patent application").
Orientation properties and durability can be improved by these proposals. In actual production of magnetic recording media, nevertheless, both the flaky particles and oxalates impair surface smoothness of the magnetic layer because the former is susceptible to particle stacking, and the latter exhibits poor dispersibility in binders.
Further, in order to achieve high-density and high-output recording, magnetic recording media are demanded to have high surface smoothness so as to minimize spacing loss in contact with a recording head. Accordingly, a lower non-magnetic layer, while not being exposed, is also increasingly demanded to have a smooth surface for coating an upper magnetic layer thereon. In addition, the influence of dispersibility in the lower non-magnetic layer on the surface properties of the upper magnetic layer simultaneously formed thereon increases with the thickness reduction of the magnetic layer. Further investigations revealed that only an improvement in dispersibility of the lower layer does not suffice for obtaining satisfactory surface smoothness of the upper layer which is simultaneously formed thereon.
A magnetic layer should have a considerable coercive force (Hc) because if a magnetic layer has a low coercive force, it suffers a great self-demagnetization loss and is not suitable for short wave recording. To this effect, it has been proposed to provide a subbing layer having a thickness of from 0.5 to 5.0 .mu.m between a non-magnetic support and a magnetic layer so that the magnetic layer may have an Hc of 1000 Oe as disclosed in JP-A-57-198536.
However, conventional techniques when applied to effecting this proposal involve problems. That is, when the technique disclosed in JP-A-57-198536 is used for simultaneous formation of such a subbing layer and the upper magnetic layer, the upper and lower layers are mixed, causing not only deteriorated surface properties but disturbed orientation. A technique for improving orientation in simultaneous coating is suggested in JP-A-3-49032, in which carbon black is dispersed in the lower layer, and orientation is conducted in multiple stages. Nevertheless, fillers having a small true specific gravity such as carbon black yield to the influence of the rotary motion of the magnetic substance during orientation, resulting in disturbance of the interface between the upper and lower layers on simultaneous coating. Thus, the above proposal, though achieving a high squareness ratio as measured in the planar direction, was insufficient for obtaining an improved residual coercive force in the direction of the normal of the magnetic layer as purposed.
An approach proposed to be taken to cope with these problems is disclosed in JP-A-62-1115. However, when the technique disclosed is applied to simultaneous coating as adopted in the present invention, the following problems arise. That is, where carbon black of low specific gravity is used in the non-magnetic lower layer, simultaneous coating or the subsequent orientation induces mixing of the non-magnetic lower layer and the upper magnetic layer or interfacial disturbances due to turbulence. Such mixing or disturbance at the interface extremely reduces orientation properties of the magnetic substance in the magnetic layer.
In case of using magnetic particles having a short major axis and a small acicular ratio, which are essentially insusceptible to flow orientation, reduction of orientation properties is conspicuous with a failure to obtain sufficient electromagnetic characteristics.
In recent years, magnetic powders to be used in a magnetic layer have been reduced in size to meet the demand for high-density recording. As the particle size decreases, the strength of the magnetic layer is so reduced. It follows, for example, the tape is stretched under high tension during preparation or running on a video deck to have increased skewness. Countermeasures against this include reduction of percent thermal shrinkage of the support or strengthening of the support, but the effect obtained is limited. Further, when a simultaneous coating system is adopted, the percent thermal shrinkage becomes greater as compared with that in the case of a successive coating system (wet-on-dry), resulting in an increase of skewness. This is because, in the latter case the lower layer is hardened after being coated by calendering or curing so that the medium is prevented from stretching, while in the former case in which the upper and lower layers are coated at once, stretching of the medium cannot be suppressed by the lower layer. Thus, the techniques disclosed in JP-A-63-187418 and JP-A-63-191315 exploiting a simultaneous coating system are accompanied by these disadvantages.
A tendency of reducing tape thickness is also developed in an attempt to extend the time of playing. Reduction in tape thickness leads to reduction in tape stiffness and, as a result, satisfactory contact with a head is impaired, resulting in reductions in electromagnetic characteristics. In particular, currently spread long-playing tapes for 8 mm-VTR or VHS have a total thickness of not more than 14 .mu.m and have a difficulty in assuring satisfactory contact with a head. With conventional thick tapes, it has been rather effective for maintenance of smooth contact with a head to reduce the strength of the lower non-magnetic layer, but the latest thin tapes used in recording and reproducing apparatus using a rotating head can hardly obtain good contact with a head unless the stiffness of the lower non-magnetic layer is increased. Use of a stretched non-magnetic support might be effective to control the lower layer stiffness but causes a reduction in stiffness in the width direction, which is unfavorable for running durability.
As described in JP-A-63-191315, although use of no polyisocyanate in a lower non-magnetic layer is recognized effective for improving contact with a head, such a medium turned out to be inferior in preservability under a high temperature and high humidity condition. Although effective in systems attaching no weight to preservability, this technique is unsuitable to systems demanding preservability, for example, for business use or for data preservation. JP-A-63-187418 also discloses thickness reduction of a magnetic layer for improving electromagnetic characteristics, but the electromagnetic characteristics attained were still unsatisfactory. JP-A-50-803 teaches use of non-magnetic pigment fine granules having a Mohs hardness of at least 6 between a magnetic layer and a support. This proposal chiefly aims at polishing of an aluminum support with a non-magnetic powder having a Mohs hardness of 6 or more to thereby increase flatness of the support.
Hence, the techniques so far developed are incapable of satisfying the demand of reducing thickness of magnetic recording media to cope with the recent trends to extension of play time and high-density recording. That is, the conventional techniques could not achieve sufficient consistency between excellent electromagnetic characteristics and running durability. In particular, to improve running durability while reducing tape thickness requires minimization of tape edge damages. From this viewpoint, the techniques of JP-A-63-191315 and JP-A-63-187418 are insufficient.
Various proposals have ever been made for obtaining magnetic recording media having an upper magnetic layer and a lower non-magnetic layer by wet-on-wet coating. For example, JP-A-50-104003 implies wet-on-wet coating but shows use of only carbon black as a non-magnetic layer, in which the layers suffer from serious interfacial disturbance due to too strong structural viscosity.
JP-A-62-212922 (corresponding to U.S. Pat. No. 4,916,024) discloses a magnetic recording medium having a conductive intermediate layer containing carbon black and a ferromagnetic powder in a proportion of from 5 to 25% by weight based on carbon black. The ferromagnetic powder is used for the purpose of improving dispersibility of carbon black. However, since the ferromagnetic powder used in the intermediate layer has equal magnetic properties, the interfacial disturbance cannot be satisfactorily prevented. JP-A-62-214524 discloses a process for producing a magnetic recording medium, in which a plurality of layers are wet-on-wet coated. This technique is characterized by selection of the formulation of each coating composition so that the solvent and solute in each layer exhibit mutual solubility with those of the adjacent layer. Combinations of an upper magnetic layer and a lower non-magnetic layer are illustrated therein, but the examples given relate only to selection of binders. Incorporation of carbon black is also suggested but failed to eliminate the interfacial disturbance. JP-A-62-241130 (corresponding to U.S. Pat. No. 4,839,225) discloses a magnetic recording medium in which the intermediate layer contains at least one binder carrying a hydroxyl group and/or an amino group and the magnetic layer contains an isocyanate compound. This technique aims at chemical bonding of the specific binder and the isocyanate compound to thereby bring about an improvement in adhesion strength between the two layers. It is described that the intermediate layer may contain carbon black, and the layers may be coated by a wet-on-wet coating system. However, the problem of interfacial disturbance could not be resolved by such disclosures.
On the other hand, JP-A-63-88080 (corresponding to U.S. Pat. No. 4,854,262) discloses a coating apparatus having an improved doctor edge. The disclosure refers to a viscosity at a high shear rate (10.sup.4 sec.sup.-1) but only showing the viscosities of coating compositions for the upper and lower layers. Such a disclosure fails to sufficiently inhibit the interfacial disturbance.
JP-A-63-146210 discloses a magnetic recording medium in which the lower magnetic layer or non-magnetic layer contains a non-curing binder and the uppermost magnetic layer contains an electron-curing binder resin. However, the illustrated lower non-magnetic layers are only those containing carbon black, and the interfacial disturbance was still unmitigated. JP-A-63-164022 discloses a method for coating a magnetic coating composition, in which multiple layers are extrusion coated through a slot die with a magnetic coating composition having a high density being sandwiched in between non-magnetic coating compositions having a viscosity lower than that of the magnetic coating composition thereby to improve high-speed thin coating properties. This coating method aims at reduction of a gap between the bead of the magnetic coating composition and the gie.beta.er. The interfacial disturbance could not be sufficiently eliminated by this technique. JP-A-63-187418 (corresponding to U.S. Pat. No. 4,863,793) discloses a magnetic recording medium in which the ferromagnetic powder of the upper magnetic layer has an average major axis length of less than 0.30 .mu.m as measured with a transmission type electron microscope and a crystallite size of less than 300 .ANG. as measured by X-ray diffractometry. The disclosure includes incorporation of carbon black, graphite, titanium oxide, etc. into the lower non-magnetic layer, referring to a specific combination of 100 parts by weight of .alpha.-Fe.sub.2 O.sub.3 and 10 parts by weight of conductive carbon. However, the amount of carbon used is small, and the particle size of .alpha.-Fe.sub.2 O.sub.3 is not specified, and the interfacial disturbance could not be sufficiently settled.
JP-A-63-191315 (corresponding to U.S. Pat. No. 4,963,433) discloses a magnetic recording medium in which the lower layer contains a thermoplastic binder and has a dry thickness of 0.5 .mu.m or greater, specifically illustrating a combination of 100 parts by weight of .alpha.-Fe.sub.2 O.sub.3 and 10 parts by weight of conductive carbon similar to JP-A-63-187418. However, the amount of carbon used is small, and the particle size of .alpha.-Fe.sub.2 O.sub.3 is not specified, and the interfacial disturbance could not be sufficiently settled.
JP-A-2-254621 discloses a magnetic recording medium, in which a non-magnetic layer mainly comprising carbon black is provided, and a magnetic layer containing Fe-Al ferromagnetic powder is wet-on-wet coated thereon. However, the illustrated example of the lower layer comprises only carbon black, which has too a high structural viscosity to remove the interfacial disturbance.
JP-A-2-257424 discloses a magnetic recording medium in which the non-magnetic layer contains a filler having an average particle size of 50 .mu.m or greater. Carbon black and abrasives, e.g., Al.sub.2 O.sub.3 and SiC, are given as examples of useful fillers. However, the specifically illustrated non-magnetic layer contains carbon black alone, Al.sub.2 O.sub.3 alone, or SiC alone. The problem of interfacial disturbance could not be resolved with such a combination.
JP-A-2-257425 discloses a magnetic recording medium containing a plurality of layers each having a coefficient of dynamic friction of not more than 0.25 and a surface specific resistivity of not more than 1.0.times.10.sup.9 .OMEGA./sq. Illustrative examples of the non-magnetic powder to be added to the lower layer are limited to SnO.sub.2 alone and carbon black alone, and the interfacial disturbance could not be avoided. JP-A-2-260231 discloses a magnetic recording medium comprising a non-magnetic support having laminated thereon a f irst non-magnetic layer, a first magnetic layer, a second non-magnetic layer, and a second magnetic layer in this order. The illustrated non-magnetic layers solely comprise binders, failing to remove the interfacial disturbance.
JP-A-3-49032 (corresponding to U.S. Pat. No. 5,051,291) discloses a magnetic recording medium whose magnetic layer has a thickness of not more than 1.5 .mu.m and a squareness ratio of not less than 0.85. while an increased squareness ratio can be obtained through multiple stage orientation, the lower layer contains only carbon black and failed to eliminate the interfacial disturbance due to its too strong structural viscosity.
In recent years, Hi 8 tapes have been given studies, ultimately seeking for obtaining merits possessed by both ME (vacuum deposited) tapes and MP (metal) tapes. It has been the most important subject to achieve such a high C/N in the short wavelength region (luminance signals in high region) as reached by ME tapes by using MP tapes while maintaining the excellent performance properties possessed by MP tapes, i.e., running properties, durability, and production suitability.
In seeking for improvements in performance of video tapes, attention has been accorded to the signal recording mechanism of VTR, i.e., recording depth of each signal, and double coating technique has been manipulated to optimize the upper and lower magnetic layers for the use intended. For example, double coating for VHS tapes has been carried out by using ferromagnetic powders different in particle size or magnetic characteristics for the upper and lower layers to realize high output and low noise in the whole region of luminance, color, and sound.
For the production of Hi 8 double-coated MP tapes, a so-called hybrid double coating system using different kinds of magnetic substances in the upper and lower magnetic layers has been developed, in which metallic magnetic substance meeting the demand of high-density recording is used in the upper magnetic layer, and iron oxide magnetic substance excellent in low region characteristics is used in the lower magnetic layer to thereby obtain high fidelity of sharp image and clear colors on reproduction.
Nevertheless, the conventional techniques or concepts have limits for the pursuit of further increased recording density and for drastic improvements in high region characteristics with Hi 8 MP tapes. Hence, the inventors have prosecuted further analyses and studies in the principles and mechanism of magnetic recording itself for the purpose of realizing a magnetic recording medium surpassing deposited tapes in high region characteristics.